1. Field of the Invention
The invention relates to a pressure-regulating valve. More particularly, the present invention relates to pressure regulating valves used in the exhaust systems of vehicles having combustion engines.
2. Description of Known Art
In exhaust systems, for example, throttle valves are known to be used as engine brakes for trucks. When used in this manner, a throttle is arranged in an exhaust-tube section such that it can be pivoted about an axis in a direction perpendicular to the longitudinal axis of the exhaust-tube section. The throttle can assume an open position, in which the throttle plane is aligned parallel to the exhaust-gas flow, and thus displays only a low level of flow resistance to the exhausts. In a closed state, the throttle is arranged such that it closes the cross section of the exhaust-tube section. It thus forms upstream, in the exhaust-gas flow, a dynamic pressure which propagates into the combustion chambers of the internal combustion engine. This pressure has a braking action there, because, with the throttle valve of the exhaust system closed, the exhaust stroke of the internal combustion engine requires a higher outlay in terms of energy.
In the case of such an engine brake, the task is essentially to build up counterpressure in a closed position and, in an open position, to allow exhaust gas to flow past as much as possible without obstruction. It is not appropriate for an engine brake to regulate pressure in the exhaust duct.
It is also known, for example for the use of exhaust-gas heat exchangers, to provide counterpressure regulating valves which are arranged in the exhaust-gas flow, through which flow takes place in the event of heating. As a result of the increased exhaust-gas pressure, the performance of the internal combustion engine is inevitably increased, and the exhaust-gas temperature is thus raised. The elevated exhaust-gas temperature produced by means of such counterpressure valves ensures an improvement in the heating capacity of exhaust-gas heat exchangers.
Furthermore, in addition to performing the pressure-regulating function, counterpressure regulating valves have to ensure that, in their open position, the larges possible throughflow cross section is available for the exhaust gas.
A valve of throttle-type construction has a large throughflow cross section in the open position because, in the open position, the flat throttle displays only a very low level of flow resistance.
WO01/50047 discloses a bypass valve which is fitted in the exhaust line and has a throttle element mounted in an eccentrically pivotable manner in an exhaust-tube section. The throttle element can be moved from an open position into a closed position via a lever and an actuating mechanism. In terms of functioning, this valve is comparable with an engine-brake valve. Defined pressure regulation does not take place by this means.
Also known is a throttle-construction pressure-regulating valve for exhaust systems in the case of which the throttle element is arranged such that it can be pivoted eccentrically about an axis and is subjected to a constant closing force and/or to a constant closing torque via an actuating mechanism.
Counterpressure regulating valves in exhaust systems have basically three operating states. A first operating state is a closed operating state, that is to say the throttle element seals the through-passage cross section completely, with the result that no exhaust gas can pass.
The second operating state is the fully opened operating state, in the case of which the throttle element has its throttle-element plane aligned in the direction of the exhaust-gas flow, with the result that the throughflow cross section is more or less fully open, that is to say is released for the through-passage of exhaust gas.
A third operating state is the so-called pressure-regulating operating state, which is described in more detail hereinbelow. In the case of this operating state, the throttle is opened partially and releases a comparatively small cross section. It is usually the case that, during pressure-regulating operation, the throttle is arranged in a state in which it has been pivoted through 0° to 5° out of the closed position.
The eccentricity of the throttle element is such that the exhaust-gas flow acts on the throttle wings of different sized adjacent to the eccentric pivot axis so as to produce an opening torque for the throttle. The opening torque on account of the exhaust-gas pressure is set counter to the closing torque on account of the constant actuating force of the actuating device. In a state of equilibrium, in the case of which the opening torque and the closing torque are of equal magnitude, the throttle element is arranged in a partially opened position, with re result that, in dependence on the closing torque applied, a constant positive pressure builds up upstream of the throttle element, as seen in the flow direction.
Such a pressure-regulating valve ensures a constant dynamic pressure merely for static flow conditions, that is to say for a constant exhaust-gas-volume flow. However, in the case of dynamic flow conditions, that is to say with the exhaust-gas-volume flow changing constantly, for example during the load cycle of an internal combustion engine, an undesirable fluctuation in pressure regulation (hysteresis) has been observed.
This is attributable to different flow and pressure conditions of the compressible exhaust gas in the gap region of the partially opened throttle. As a result, in the case of increasing exhaust-gas flow, a dynamic pressure is established upstream of the throttle that differs from the case of a dynamically decreasing exhaust-gas flow. Such occurs during each load cycle of an internal combustion engine.